Coal continues to be a prevalent source of energy, particularly for electric generating stations. Millions of tons of coal are transported by railway cars each year within the United States between a coal mine and a user facility. In order to maintain the condition of the railways and to insure safety, regulations are imposed to limit the total weight of each railway car. Currently, the total weight allowed for each railway car, including the weight of the car itself, is 286,000 pounds. As the coal companies obviously desire to ship the most amount of coal per railway car or ship a certain amount of coal with the least amount of railway cars, there has been a continuous effort to lower the weight of the railway cars while still maintaining the railway cars' structural integrity. However, one limiting factor in improving railway cars is the fact that as the coal is transported across the country, it may pass through various states and be subjected to various weather conditions. For example, when coal passes through sub-zero temperatures, it will freeze, and thus require the coal to be subjected to a thawing process. Thus, any components in the coal cars must be able to withstand the various stresses imparted upon them.
Heretofore, it has been known to use an aluminum door with a steel frame for rapid discharge coal car doors. The use of an all metal door, especially with a steel frame, results in a door having a weight of almost 190 pounds. Heretofore used all-steel doors are even heavier. Most coal cars are hopper cars with multiple outlets in the bottom selectively opened and closed by discharge doors. A commonly found hopper car has sixteen discharge doors working together in pairs to cover eight discharge outlets. The total weight of sixteen all-metal doors is over 3000 pounds, or about one percent of the total allotted weight per railway car.
Others have considered using certain plastics, such as polyurethane, as a substitute for the aluminum doors. However, such plastic doors generally would not withstand the heating the doors must be subjected to during the thawing process. Further, it is unclear whether doors made of regular polyurethane would have sufficient strength to adequately support the coal load and perform adequately as a rapid discharge coal car door.